Steven Plisk - Yale University, Connecticut, USA Introduction
The need for strength and endurance in sports is accepted by most coaches and
athletes. In spite of evidence for direct transfer to performance and injury
prevention, the role of strength and endurance is often perceived as indirect or
foundational. Indeed, strength is often incorrectly thought to be independent
from (or incompatible with) movement velocity, when in fact the latter is a
result of force application. Considering that technical precision and speed of
execution are fundamental goals of sports, this realization is crucial in order
to achieve optimal training effects and performance.

Sports performance is typically determined by the ability to execute skills and
assignments at a planned effort level. Training tasks should therefore be
selected and prioritized according to their specific relationships to the
coordinative, biomechanical and bioenergetic demands of competition. In general,
maximum strength and speed-strength training should be conducted with limited
work volumes and minimal metabolic stress in order to maximize the quality of
learning and training effects (although hypertrophic methods associated with the
former are an exception). Likewise, strength-endurance training usually involves
fatiguing workloads and greater overall volume. As part of the overall sports
preparation process, specialized strength training should be planned and
implemented according to sound principles in order to optimize the athlete's
performance capabilities.

Movement Mechanics
Effective strength training begins with a working knowledge of basic movement
mechanics, especially with regard to: rate of force development (RFD) and
impulse; stretch-shortening cycle (SSC) and reactive ability; power; and their
roles in "endurance" vs. "power" sports. The operative concept in each case is
speed-strength - i.e. the ability to develop forces rapidly and/or at high
velocities. Collectively, this section illustrates that evaluation of an
athlete's explosive and reactive strength capabilities is the starting point for
planning the preparation process. Fortunately, such tests are relatively simple
to administer and interpret.

RFD & Impulse
The brief execution times of most athletic tasks require high RFD. Case in
point: force is applied for 0.08 - 0.2 sec during the ground support phase of
running, whereas peak force production requires up to 0.6 - 0.8 sec in dynamic
movements (and up to 3-4 sec in isometric movements). Even in largely
non-ballistic locomotion such as cycling, rowing, or swimming, performance is
usually determined by the ability to generate force quickly and thereby achieve
a critical impulse output (i.e. the change in momentum resulting from a force,
measured as the product of force and time; Figure 1).

Isometric force as a function of time, indicating maximum strength,
rate of force development [RFD], and force at 0.2 sec for untrained [solid
line], heavy-resistance trained [dashed line], and explosive-ballistic trained
[dotted line] subjects. Impulse is the product of force and time, represented by
the area under each curve; and is increased by improving RFD. Source: Newton
Kraemer. Adapted from: Häkkinen & Komi Scandinavian Journal of Sports Science
7(2): 55-64 & 65-76, 1985

The practical implication of this is that amplitude, direction and rate of force
application are equally significant when performing functional tasks. As will be
discussed in more detail in the Exercise Prioritization & Substitution section,
the issue is one of specificity to competitive demands. Thus, a basic objective
of training is to improve RFD, effectively moving the force-time curve "up and
to the left" and thereby generating greater impulse during the limited time (and
distance) in which force is applied. Furthermore, the significance of this
parameter is not limited to biomechanics: According to the "impulse-timing
hypothesis", the force-time relationship is a central component in motor
programming and has important implications for motor control and learning.

SSC & Reactive Ability
Many explosive movements can involve the reflex/elastic properties of the
muscle-tendon complex and are ballistic in nature, even when initiated from a
static position. The action begins with a preparatory countermovement where the
involved muscles are rapidly and forcibly lengthened or "stretch loaded", and
immediately shortened in a reactive or elastic manner. This eccentric-concentric
coupling phenomenon - referred to as the stretch-shortening cycle (SSC) - is
often observed in sports, particularly those involving running, jumping and
rapid changes in speed and direction. SSC actions exploit motoneural reflexes as
well as intrinsic qualities of the muscle-tendon complex. Although the exact
relationship of reflex/elastic properties of the muscle-tendon complex to
maximum strength is not completely understood there does appear to be a degree
of independence, thus training for maximum strength will not optimally train
these properties. Training for such sports should therefore progressively
include "plyometric" methods in addition to basic "heavy resistance" movements.

It is important to distinguish the concept of reactive ability from that of
reaction time. The former is a characteristic of speed-strength exhibited in
stretch-shortening cycle (SSC) actions which can be improved through
reactive-explosive training (graphically, this can be illustrated as a rapid and
efficient transition from eccentric muscle action in the lower quadrant of
Figure 2 to concentric muscle action in the upper quadrant). In contrast, the
latter is a relatively un-trainable quality which correlates poorly with
movement action time or performance in many brief explosive events. For example,
an elite sprinter's auditory reaction time typically ranges from 0.12 - 0.18sec,
but is not significantly related to his/her 100 m results. Other factors such as
acceleration, speed-endurance and (to a lesser extent) maximum speed are more
closely associated with overall sprint times. Reaction time is, however, an
important determinant of performance in quick timing tasks (e.g., a batter
hitting a baseball) and defensive types of stimulus-response actions (e.g., a
goaltender making a save).

Power
The peak levels of force and power (energy) absorbed by the tissues while
actively lengthening are often greater than those produced while shortening
(Figure 2). If not adequately addressed in training, this can be the mechanism
of so-called non-contact injury, technical inefficiency or outright
non-athleticism. Thus, in addition to improving concentric power production
capability, the demands of SSC movements dictate two more training objectives:
to develop the eccentric strength needed to tolerate extreme power absorption
while explosively braking during the initial lengthening action, as well as the
reactive strength needed to rapidly recoil into the subsequent shortening
action.

also illustrates that achievable movement speed also depends on the
load to be overcome. Simply put, as resistance increases in any task, so does
the role of strength in determining velocity or acceleration. In terrestrial
movement this resistance usually includes the athlete's own body mass, and
possibly his/her equipment or opponent (in comparison, despite the fact that
aquatic locomotion is not "weight bearing", consider the challenge presented by
hydraulic resistance - i.e. energy cost as a function of drag). Indeed, it is
difficult to find examples of sports where power and high-speed force output are
not required to rapidly accelerate, decelerate or achieve high velocities.
Primarily the athlete's percentage of type II motor units, and the ability to
optimally activate them determine these capabilities. In contrast, isometric or
low-velocity maximum strength is a function of muscle cross-sectional area (i.e.
the number of active sarcomeres in parallel). Once the upper limit for specific
muscle tension can be achieved (40 - 45 N/cm2 in trained athletes), hypertrophy
is required - especially in type II fibers - in order to further increase force
and speed production.

'Endurance' vs 'Power' Sports
It is generally accepted that these speed-strength capabilities are important in
"power" sports involving explosive running, jumping and changes in speed or
direction. There is a common misconception, however, that their role in
"endurance" types of activities is minor. The ability to apply force rapidly and
accelerate one's body mass is the rule rather than the exception in athletics.
While prolonged activities certainly require specialized metabolic capacity,
they often involve a series of brief, explosive "spikes" in power output. Thus,
the simplistic classification of endurance events as sub-maximal or non-strength
related should be reconsidered. The task-specific importance of speed-strength
should be critically evaluated on a mechanical basis, rather than categorically
dismissed for physiological reasons.

Systems vs Components
In general, structural movements (e.g. multi-joint, weight-bearing exercises)
have a systemic effect which reaches far beyond the muscle fibers used in their
execution. Muscles act - and must be targeted - in functional task groups rather
than in isolation. This is one reason why athletes are well advised to emphasize
powerlifting and weightlifting-style movements, and compound exercises in
general, in their training. Furthermore, such movements are also a potent means
by which the neuromuscular and neuroendocrine systems are activated, in turn
up-regulating every system in the body. Thus, there are several reasons why
strength training programs should be based on free-weight movements rather than
isolated muscles:

Power The greater the effort - and acceleration - with a given weight, the
greater the power development and subsequent training effect. Peak power output
during weightlifting movements (snatch and clean & jerk) is the highest ever
documented, and is comparable to the maximum theoretically possible for a human.
For example, the explosive "jump and pull" or "dip and drive" actions of these
movements are executed in 0.2 - 0.3sec; and peak power production is:

4-5 times that of the deadlift or squat
11-15 times that of the bench press
Motor Coordination Skillful movements have a motor control / learning effect
which carries over to the athlete's "coordinative abilities":

orientation
differentiation
reactive ability
rhythm
balance
combinatory ability
adaptive ability
Systemic Effect To a point the greater the exertion, especially in multi-joint
lifts, the higher the production of most endogenous hormones. Some evidence
indicates that an increased blood concentration of anabolic hormones, such as
somatotropin or testosterone, in response to resistance exercises are involved
in stimulating overall changes in muscle mass and strength. The following
guidelines are not definitive, and there probably is no ideal workload protocol
for either effect. However, a sound training strategy must account for and
exploit such basic adaptive mechanisms:

Moderate weights for higher reps (8-10/set), and high-intensity endurance
activities in general, maximize the somatotropin and testosterone response.
Large muscle mass exercises (i.e. multi-joint) performed for multiple sets
result in greater hormonal respsonses compared to small muscle mass exercises.

There is nothing magical about the sound of iron clanking, and in fact certain
machines such as a hip sled or cable-pulley system can serve useful roles. But
there is an inherent advantage to multi-joint free weight training which cams,
levers or linear bearings cannot match: It requires - and develops - functional
strength, and has excellent transfer to athleticism and explosiveness.

Exercise Prioritization
In terms of specificity, training tasks should be selected and prioritized
according to their dynamic correspondence with the demands of the activity (also
referred to as specificity or transfer of training effect): Their basic
biomechanics - but not necessarily outward appearance - should be specific to
those occurring in competition. The rate of force development and time of force
production (impulse; Figure 1) and dynamics of effort (power; Figure 2) are
especially important criteria in explosive athletic movements. Other practical
considerations include amplitude and direction of movement, accentuated region
of force application, and regime of muscular work.

This concept is analogous to the motor learning principle of practice
specificity with respect to sensorimotor, processing and contextual effects on
acquisition, retention and transfer. While these may appear to be statements of
common sense, it is difficult to overstate their importance because failure to
address them in training can result in limited transfer to competitive
performance.

Program Design
In order to maximize the athlete's performance capabilities, the sports
preparation process must be planned and implemented according to sound
principles. With regard to specialized strength development, the following
variables must be rationally manipulated:

Action speed the intent to accelerate and/or achieve high velocity with a given
load as a means of manipulating power or impulse production
Exercise order the sequence in which a set of exercises is performed
Density the amount of work performed in a set or training session
Frequency the number of training sessions performed in a given time period
(e.g., day or week)
Intensity the effort with which a repetition is executed (usually characterized
by resistance, but more accurately associated with impulse or power output)
Recovery the time interval between sets
Repetition the execution of a specific workload assignment or movement technique
Series a group of sets and recovery intervals
Set a group of repetitions
Volume he amount of work performed in a given training session or time period
(usually characterized by the volume load (repetitions x mass lifted), but more
accurately associated with the product of resistance and distance moved per
repetition)
These parameters are useful in quantifying training, and in most cases can also
be adapted or directly applied to speed, agility and speed-endurance
development. In order to be useful in practice, however, they must be
accompanied by qualitative guidelines regarding movement mechanics and planned
variation in training objectives.

Summary
There is tremendous potential to improve an athlete's performance capability and
minimize the risk of injury through specialized strength training.
Principle-based planning and implementation of the preparation process is the
key. This requires a working knowledge of physiological and biomechanical bases
of maximum strength, speed-strength and strength-endurance development.

In conclusion, the following practical implications can be recommended:

Explosive force application is the basis of strength training for sports
Functional strength is expressed in terms of acceleration, execution time or
velocity - especially in athletics. Training tactics which disregard this fact
are fundamentally unsound. Moving through an acceleration path, and applying
rapid and/or high-speed force, is the name of the game.
Emphasize big basic movements which have the greatest training effects; and use
equipment which challenges the athlete to control, direct and/or stabilize it
Muscles act in functional task groups, and must be targeted via force
transmission through (rather than isolation within) the body's "kinetic chain".
Multi-joint free weight movements are superior in this regard.
Distinguish between specificity and simulation Training tasks should be selected
and prioritized according to the coordinative, biomechanical and bioenergetic
demands of competition.
Balance the need for specificity vs. variability Maintain stability in the
program by sticking with a basic exercise menu rather than trying to include
every possible movement. Variation can be achieved by cycling workloads on a
"periodic" 3-4 week basis in order to summate their training effects and avoid
accommodation problems.
Quality, not quantity, of effort is the bottom line While it is necessary to do
enough work to get a training effect, there is likely a threshold of diminishing
returns above which the athlete's effort is diluted - and recoverability /
adaptability are compromised. Fitness and fatigue are a trade-off beyond a
certain point. Generally, the best results are achieved by maximizing the
quality of effort within a prescribed amount of work.
Effort and recovery are interdependent The interrelation of workload, intensity,
frequency and volume cannot be changed arbitrarily. They must be adjusted
together, which occurs automatically with a sound plan. A training program is
only as good as the athlete's ability to recover from and adapt to it!
Fitness qualities are means toward an end, not ends in themselves: to develop
the athlete's performance capabilities and skills, and thereby couple effort
with execution Power, flexibility, agility, speed and endurance - combined with
motor coordination - are the elements of athleticism. Each part can be trained,
but they must be trained collectively because they are parts of a larger whole.
None is a separate entity, nor more important than another. Train athletes, not
muscles!
Most importantly, skillful tasks are the basis of sports training, and require
the services of a qualified Strength & Conditioning coach If simply counting
reps and sets were the answer, anyone could do it. As is the case in all aspects
of coaching or teaching, attention must be directed toward what the athlete is
doing as well as how they are doing it - not just how much they do. Skilled
training requires skilled coaching, and without it the program isn't worth the
paper it's written on. Key Terms
Dynamic Correspondence A principle stating that the basic biomechanics - but not
necessarily outward appearance - of training tasks should be specific to those
occurring in competition (analogous to the practice specificity concept).
Impulse The change in momentum resulting from a force, measured as the product
of force and time; an important criterion of speed-strength.
Strength The ability to apply force in brief maximal efforts or repeated
submaximal efforts.
Plyometrics Explosive-reactive training comprised of stretch-shortening cycle
actions; aimed at improving explosive-reactive qualities of strength. Acute
responses include increased mechanical efficiency and working effect (e.g.,
power, rate of force development), while chronic responses involve up-regulated
muscle stiffness and motoneural activation.
Power The rate of doing work, or product of force and velocity; an important
criterion of speed-strength.
Practice Specificity A motor learning principle stating that the contextual,
processing and sensorimotor effects of training tasks on acquisition, retention
and transfer should be specific to those occurring in competition (analogous to
the dynamic correspondence concept).
Reactive Ability A characteristic of speed-strength (e.g., in stretch-shortening
cycle actions) which can be improved through plyometric training aimed at
improving explosive-reactive strength.
Speed-Strength The ability to apply force rapidly and/or at high velocities,
i.e. in submaximal accelerative efforts or reactive-ballistic efforts; usually
expressed as impulse or power.
Strength-Endurance The ability to maintain force in extensive/intensive interval
workloads or repeated submaximal efforts.
Stretch-Shortening Cycle Muscle actions characterized by impulsive
eccentric-concentric coupling (i.e. rapid deceleration/lengthening immediately
followed by acceleration/shortening in the opposite direction) which exploit
reflex potentiation and elastic energy recovery phenomena. Such actions are
intrinsic to functional - and especially athletic - movement, and are the basis
of plyometric training aimed at improving explosive-reactive strength